It's going to be hard to discuss this here, since we can't name names.
Please, please, do NOT use FTP in any case. FTP is not a secure protocol.
sFTP (Secure FTP) is secure, but actually has nothing to do with FTP. It runs over SSH.
If you are running Windows, there are some products that will allow you to map a drive letter to an sFTP site. (Or, in most cases, to an FTP site as well - but DON'T DO IT!)
Beyond communication security, see if the provider encrypts the data that is stored on their hard drives. And you probably don't want to go with a nobody.
Frankly, it sounds like you need something more than what you are asking for, in any case. This is "collobrative authoring". I'd start by doing a search on that term.
In most cases, users will want to know just who changed what and when. That kind of thing could certainly be (and is) done with a secure website (SSL) running collobrative authoring software.
If they've got the bucks (they're lawyers - they do!) I'm sure there are such systems specifically aimed at law practices.
Simply having people upload and download files willy-nilly is poor way to do collaborative authoring. I could see 2, maybe 3 people working this way. But not a whole law office.